1. Field
The present description relates to the field of patterning microelectronic and micromechanical devices, and in particular, to forming patterns using an array of probe tips to indent a photoresist or mask layer.
2. Related Art
To increase the number of transistors, diodes, resistors, capacitors, and other circuit elements on an integrated circuit chip, these devices are placed closer and closer together. This requires that each device be made smaller.
Current manufacturing technologies use laser light with a wavelength of 193 nm for photolithography. These are referred to as Deep Ultraviolet (DUV) systems. These systems which are the workhorses of 65 nm lithography in 2006 are being continuously improved and are projected to be used to produce features as small as 20 nm. However, at this point, it is anticipated that a different technology must be applied.
One obstacle to producing still smaller features is the wavelength of the light being used. The next step that has been proposed is to use light of 13.4 nm, or other wavelengths in the range of 4 nm-30 nm, referred to as Extreme Ultraviolet (EUV) light. Depending on the rest of the system and process parameters, this light may allow features to be created that are less than 20 nm across and probably as small as 10 nm or less across.
The smaller size of the printed features is a result of the improvement in resolution. The resolution of a photolithography system is proportional to the wavelength of the light divided by the numerical aperture of the illumination system's projection optics. As a result, the resolution can be improved by either decreasing the wavelength of the light used, or by increasing the numerical aperture (NA) of the photolithography projection optics, or both.
As the wavelength of the illumination light gets smaller, the lithography process becomes more difficult and more expensive. One popular wavelength for proposed EUV photolithography is 13.5 nm. All known materials absorb light at this frequency. As a result, the projection optics cannot be made using transparent lenses and the best mirrors so far developed reflect only about 70% of the light that shines on them. The other 30% of the light is absorbed by the mirror. This compares poorly with even poor quality visible light mirrors that reflect more than 95% of the light that shines on them. The absorption of EUV light also produces unwanted heat and wear on any materials that may be used in an EUV photolithography process, included the materials in the mask.
In addition, the masks proposed for EUV are to be fabricated with E-Beam lithography. The combination of the difficult EUV light and E-Beam patterning is expected to be significantly more costly than the current optical methods.
Another difficulty is the high speed desired in semiconductor manufacturing to keep costs down and volumes high. A modern photolithography plant can pattern hundreds of wafers per hour. Alternative approaches to photolithography include imprint and direct write lithography with charged particle beams. The imprint lithography requires masks or stamping tools that are the same size as the features to be patterned. These are expected to be very hard to make without some defects. Direct write lithography is a serial writing approach using, for example, an E-beam and is expected to be too slow for high volume manufacturing.